In die casting, molten metal is forced into the die from the hydraulic cylinder by a piston-type plunger or ram displaced over a linear stroke under controlled operating conditions. Research into die casting methodology has achieved a level of sophistication whereby the proper injection speeds, pressure (including vacuum pressure), and temperatures required for optimum operation for any given machine can be theoretically calculated. To satisfy the calculated conditions, however, the operations must be carried out within a relatively narrow range of settings for each of the critical process parameters. Variation in one or more of the process parameters can often affect the performance of the other parameters on the process and may alter the production rate, yield and/or quality of the product produced.
Alternatively, where process variation is inevitable or expected, by predicting part quality from the objective measurements, parts may then be segregated. Such a process is one feature in reducing the total cost for a production facility in producing a given product. In fact, substandard parts are preferably removed early in production before value added operations add more cost to a product that can not be sold. Such operations include, but are not limited to, drilling and tapping holes, or the machining of features.
One of the parameters that is subject to variation is the amount of gas present in the final product. Increasing concentrations of gases in a cast product directly correlate with increased porosity of the product which, in turn, can compromise part integrity. One approach to reducing the aforementioned variation has been to employ vacuum systems to assist in the removal of unwanted gases from a die cavity and shot sleeve prior to and during injection of molten metal. However, particularly where die cast parts are mass produced, inconsistent and/or inadequate vacuum systems can result in unacceptably wide variations in part quality. Such variety in part integrity then require additional measures for quality control which can be costly and time consuming.
Traditionally, to optimize the production output the process parameter settings are adjusted over a series of production runs until a product with desired characteristics and production yield is obtained. Since often times the events which affect the production cycle occur much too quickly for human observation it was necessary, heretofore, for operating personnel to make subjective judgments in adjusting process parameters in a production run frequently resulting in high scrap rates.
The trial and error technique commonly used heretofore also required more raw material than necessary in getting a given required production output. Moreover, because of the inability to precisely control and adjust the process parameters accurately, the design of the article produced typically had to be made with more material than necessary for functional or strength considerations simply to permit an acceptable yield.
Accordingly, a need exists for a diagnostic instrumentation system which can readily be applied to the process machinery to monitor and record objective measurements of machine performance during an actual production cycle. The objective measurements should represent machine operational data which can provide technical personnel with the means to make appropriate adjustments to not only maintain optimum process integrity but also predict part quality based on the measurements.
As the vacuum applied prior to and during casting of a product can be indicative of part quality, it would be desirable to provide a method and apparatus for measuring the vacuum in die casting machinery. It would also be desirable to apply vacuum consistently from one part to the next based on measured reading of the vacuum. It would be further desirable to provide a method and apparatus for segregating cast parts based on vacuum measurements taken before and during casting of the respective part.